A fiber-optic patch cord is constructed from a core with a high refractive index, surrounded by a coating with a low refractive index, that is strengthened by aramid yarns and surrounded by a protective jacket. Depending on the patch cord's application, the buffer coating can be a variety of different materials that will offer, for example, resistance to high temperatures or fire. A fiber optic patch cord (fiber jumper) is: Typical applications: A patch cord is the “bridge” that connects two fiber devices and lets them talk to each other. Their performance directly impacts signal quality, insertion loss (IL), and return loss (RL). Key functions of a fiber patch cord:.